THE WORK OF SNOW AND ICE. 283 



than the ice, and the latter rides over it. Superglacial material may 

 be acquired during movement by the fall of debris from cliffs, or by 

 the descent of avalanches. 



Conditions influencing rate of erosion. — An obstructive attitude of 

 the surface toward the movement of the ice is as necessary for effective 

 erosion as the movement of the 

 ice itself. Advancing over a flat 

 surface, ice ordinarily inflicts but 

 little wear, since there is little 

 for it to get hold of. So slight is 

 the abrasive power of ice under 

 these conditions that it fre- 

 quently overrides and buries the Fig. 252.— Diagram showing lack of contact! 

 soil with more or less of its of the edge of the ice with its bed. I 



herbaceous vegetation. But while a certain measure of roughness 

 of surface is favorable for glacial erosion, the topography may be so 

 uneven as to seriously impede the ice. Erosion is probably at its 

 maximum, so far as influenced by topography, when the roughness 

 of the surface is such as to offer notable catchment for the basal ice, 

 but not such as to impede its motion very seriously. The amount of 

 rehef favorable for the greatest erosion increases with increasing thick- 

 ness of the ice. 



Other conditions which influence erosion by ice are (1) the amount 

 of loose or slightly attached debris on the surface, (2) the resistance 

 of the rock, (3) the slope of the surface, (4) the thickness of the ice, 

 (5) the rate of movement, and (6) the abundance and character of 

 the debris which the ice has to work with. The effect of the first five of 

 these conditions is evident. The effect of the last is less simple. Clean 

 ice passing over a smooth surface of solid rock has little effect upon it; 

 but a rock-shod glacier will abrade the same surface notably. The 

 effect of this abrasion is shown in the grooves and scratches (strice) 

 which the stones in the bottom of the ice inflict on the surface of the 

 rock over which they pass (Figs. 253, 255, and 256) . At the same time the 

 stones in the ice are themselves worn both by abrasion with the bottom, 

 and with one another (Fig. 254). It does not follow, however, that 

 the more material in the bottom of the ice the greater the erosion it 

 effects; for with increase of debris there may be decrease of motion^ and, 

 ^ Russell. Jour, of Geol, Vol. Ill, p. 823. 



